RE: Discharged Li-Po
5.21 Volts? There's not much hope of reviving that pack. Once you get below 7.5 Volts on a 3S pack, the writing is pretty much on the wall. I would also suspect that the cells in the pack are now horrendously out of balance, if one cell isn't dead outright. Trying to charge the pack, even slowly with a wall wart, is potentially dangerous at this point.